Clay Science 23, 15–18 (2019)–Note–ULTRASONIC VELOCITY MEASUREMENT INIMOGOLITE-BASED COLLOIDAL SOLUTION AND
THIXOTROPIC HYDROGEL
Kazuhiro Shikinakaa, *, Wei Qiub, †, Neisei Hayashic,
Yosuke Mizunob, and Kentaro NakamurabaResearch Institute of Chemical Processing Technology,
National Institute of Advanced Industrial Science and Technology,
4–2–1 Nigatake, Miyagino-ku, Sendai, Miyagi 983–8551, Japanb
Institute of Innovative Research, Tokyo Institute of Technology,
4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226–8503, Japanc
Research Center for Advanced Science and Technology, The University of Tokyo,
4–6–1 Komaba, Meguro-ku, Tokyo 153–8904, Japan(Received November 27, 2018. Accepted December 13, 2018)

ABSTRACTThe ultrasonic velocities in a colloidal solution and a thixotropic hydrogel containing imogolite are measured.
Our original experimental setup can detect the change of ultrasonic velocity caused by the mixing of imogoliteand maleic acid in pure water. Using this setup, the ultrasonic velocity is found to remain unchanged during themacroscopic solid/liquid transition and viscoelastic change of a thixotropic hydrogel consisting of imogolite andmaleic acid, which is consistent with previous literatures. Thus, the ultrasonic velocity measurement proves tobe effective for the detection of well-dispersed clay minerals in diluted state, regardless of the phase and viscoelastic change of substances.
Key words: imogolite, ultrasonic waves, hydrogel, thixotropy, phase transition

INTRODUCTION
Ultrasonic testing, which is one of the most frequently
used nondestructive testing techniques, has been extensively
employed for internal ﬂaw detection and material characterization (Krautkrämer and Krautkrämer, 1990). In particular,
evaluation of ultrasonic velocities has been widely used toinvestigate material properties such as elastic constants, porosity, and grain size in solids (Hübschen, 2016), and density
and compressibility in liquids (Povey, 1997). Consequently,
this technique has been used for the characterization ofclays. To date, the properties of shale (e.g., pore size distribution) containing different types of clay minerals (Kuila andPrasad, 2010) have been measured using dry samples (Vanorio et al., 2002 and Gong et al., 2018).
* Corresponding author: Kazuhiro Shikinaka, Research Institute ofChemical Processing Technology, National Institute of Advanced
Industrial Science and Technology, 4–2–1 Nigatake, Miyagino-ku,
Sendai, Miyagi 983–8551, Japan. e-mail: kaz.shikinaka@aist.go.jp†
Present Address: Department of Physics, Technical University ofDenmark, DTU Physics Building 309, DK-2800 Kongens Lyngby,
Denmark.
doi: 10.11362/jcssjclayscience.23.1_15

On the other hand, since clay minerals are utilized ascomponents in functional solutions and hydrogels (Shikinaka
et al., 2014a, b), the measurement of their properties in diluted
state is also essential. However, the current techniques, which
include transmission electron microscopy and synchrotron
X-ray scattering, are complicated to perform and induce denaturation of the samples (Bergaya et al., 2006). To solve these
problems, we propose an easy and noncontact technique forthe detection of clay minerals in diluted state that is based on
ultrasonic velocity measurement, which is able to estimate thepresence of substances in its solution.
In this study, we demonstrate this idea by measuring theultrasonic velocity in a colloidal solution and a hydrogel containing imogolite. Imogolite is an aluminosilicate clay mineral that consists of rigid single-walled cylindrical nanotubes
of composition (HO)3Al2O3SiOH (Farmer et al., 1977, 1983).
Due to the presence of many hydroxyl groups such as alminol(outer) and silanol (inner) on its surface, imogolite nanotubes
disperse well in polar solvents such as water. The imogolite
nanotube has a high aspect ratio, i.e., its external diameter isabout 2 nm and its length is several tens of nanometers to several micrometers. Recently, imogolite nanotubes were crosslinked by hydrogen bonding via dicarboxylic acids, causing
gelation of an imogolite-based colloidal solution (Shikinaka

K. Shikinakaa et al.

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et al., 2014a). Moreover, a homogenous gel consisting ofimogolite and maleic acid showed hysteresis-free thixotropic
nature, owing to the characteristics of imogolite as perfectly
rigid polyelectrolyte (Donkai et al., 1985) at a certain mixing ratio of aluminol groups in imogolite to carboxyl groups
in maleic acid such as 1 : 1. Herein, we measure the changeof ultrasonic velocity in a colloidal solution and a hydrogel
containing imogolite to detect well-diluted clay minerals insolvent in a noncontact manner.
MATERIALS AND METHODS
Deionized water further puriﬁed using a Milli-Q® Advantage A10® system (MilliporeTM, Eschborn, Germany) wasused throughout the experiments. Other reagent-grade chemicals, except for imogolite, were purchased from Tokyo Kasei
Chemicals and Wako Pure Chemical Industries and used asreceived.
Imogolite was synthesized and puriﬁed according to themethod described in Farmer et al. (1983) and Shikinaka et al.
(2015) as follows. Aqueous solutions of AlCl3·6H2O (9.96 g in
369 mL of puriﬁed deionized water; Kanto Chem. Co. Inc.,
Japan) and Na4SiO4 (6.90 g in 362 mL of puriﬁed deionizedwater; Junsei Chem. Co. Ltd., Japan) were mixed to prepare asolution containing 12.5 and 2.5 mol L−1 of Al and Si, respectively. The pH of the mixture was adjusted to 6.0 by rapidly
adding ca. 26 mL of 1.0 mol L−1 NaOH aqueous solution (thesolution was stirred to avoid the occurrence of locally highpH). The resulting solution was stirred for 1 h. The resultant
white precipitates were collected by centrifugation and werethen redispersed in 400 mL of water under stirring. After adding further 2400 mL of water, the solution pH was adjusted to
4.5 by adding 7–8 mL of 1.0 mol L−1 HCl. The solution wasthen carefully and continuously heated for 4 days at 100°C
under gentle stirring. The as-prepared imogolite solution wasthen cooled to room temperature, and a ﬁne powder of sodium
chloride (16.4 g) was added under stirring. The resulting gel
was collected by centrifugation (5000 rpm, 30 min) and was
subsequently washed portionwise with 500 mL of water using a 100 nm Millipore ﬁlter under suction. The wet products(caution should be taken to stop the samples from drying out)
were added to 1800 mL of tetrahydrofuran (stabilizer-free
grade) under stirring, and the ﬂuffy precipitates were collected by ﬁltration and dried in vacuo, resulting in a yieldof 42%. For the preparation of the imogolite-based colloidal
solution, a calculated amount of imogolite in pure water (forpreparing 6.4 wt% colloidal solution) was sonicated for 4 h
at 100 W (FU-21H, SDUltra Ltd., Korea), while maintaining
room temperature by intermittent addition of ice to the sonicator bath. Through this procedure, slightly opaque solutionsof 0.2 mol L−1 aluminol functional groups were obtained, andthe average length of imogolite nanotubes in the solution
was shortened to 131 nm, which was conﬁrmed by transmission electron microscopy (Shikinaka et al., 2015). For the
0.08 mol L−1 imogolite aqueous solution (1 equivalent withrespect to the –Al(OH)2 group), the same volume of aqueous
solution of 0.08 mol L−1 maleic acid was added under stirring
to obtain a hydrogel (Shikinaka et al., 2014a).
The ultrasonic velocity in a solution was measured using

Fig. 1. Sketch of the experimental setup for measuring ultrasonicvelocity. AMP: ampliﬁer, FG: function generator, LPF: lowpass ﬁlter, OSC: oscilloscope.

our original setup shown in Figure 1. This setup is an improved version of our previously reported setups (Hayashi
et al., 2011, 2012, Minakawa et al., 2013). Using a pipette, thesample solution was injected into an acrylic cylindrical container with a height of 11.0 mm, an inner diameter of 6.0 mm,
and a thickness of 1.0 mm. The room temperature was 20°C.
Before a thixotropic gel (i.e., imogolite–maleic-acid gel)was employed as the sample, it was well shaken to ensure it
maintained its liquid state. Using a function generator and anelectrical ampliﬁer, a rectangular pulse voltage (peak voltage:
10 V, repetition rate: 100 Hz, duty ratio: 0.3%) was generated
and applied to a vibrator A (transmitter; resonance frequency:
ca. 357 kHz) attached to an aluminum bar (with a step hornused to amplify the vibration). The vibration generated at thevibrator A was transmitted to the bottom of the bar, and theultrasonic wave was emitted and propagated in the solution.
When the ultrasonic wave reached a vibrator B (receiver)attached to the bottom of the container, the corresponding
voltage was output from the vibrator B. After noise suppression by low-pass ﬁltering (cut-off frequency: 1.59 MHz), theoutput signal was observed using an oscilloscope. The propagation time ta, which is deﬁned as the time required for theultrasonic wave to reach the vibrator B, is not inﬂuenced by
the multiple reﬂections inside the solution and the bottom of
the container. Therefore, by measuring ta while changing therelative position d of the vibrator A (note that d was deﬁned
to be positive when the vibrator A was far from the vibratorB), the ultrasonic velocity in the solution can be determinedas Δd/Δta, where Δd and Δta indicate the changes in d and ta,
respectively.
RESULTS
Figure 2 shows the output waveforms obtained when

Ultrasonic measurement in imogolite-based colloidal solution and hydrogel17

Fig. 2. Output waveforms of the receiver obtained at different vibrator positions d when the ultrasonic pulses transmit throughthe imogolite–maleic-acid gel.
Fig. 4. Output waveforms of the receiver when ultrasonic pulses
transmit through the imogolite–maleic-acid gel at liquid state(t=3 min, dashed curve) and gelated state (t=3.4 h, solid curve).
Table 1.

Fig. 3. Calculation of ultrasonic velocity from the dip shift dependence on the vibrator displacement d by linear ﬁtting.

the relative positions d of the vibrator A were 0.0, 0.2, and
0.4 mm, respectively, when using the imogolite–maleicacid gel (Shikinaka et al., 2014a) as the sample. Clear dips
(indicated by red circles) indicated the ﬁrst detection of theultrasonic wave. As expected, the appearance of the dip wasdelayed as d increased. The dependence of the dip shift on thedisplacement d was well ﬁtted by a linear trend (Figure 3),
and the inverse of the slope of this trend gives the ultrasonicvelocity in the solution. Thus, the ultrasonic velocity V in theimogolite–maleic-acid gel (liquid state) was calculated to be
1,522 m s−1.
In the same manner, the ultrasonic velocities V of pure water, imogolite-based colloidal solution, maleic acid solution,and imogolite–maleic-acid gel were measured, and the corresponding results are summarized in Table 1. The measured V

Measured ultrasonic velocities V of various solutions.
Solution

V [m s−1]

Pure water
Imogolite colloidal solution
Maleic acid solution
Imogolite–maleic-acid gel

1,499
1,460
1,603
1,522

value of pure water (1,499 m s−1) was in good agreement withthe theoretical and measured values found in the literature(Lubbers and Graaff, 1998), which proves the correctness andaccuracy of our setup for measuring the ultrasonic velocity.
After imogolite was mixed, V decreased (1,460 m s−1) probably because of the high crystallinity of imogolite relative towater. In contrast, the maleic acid water solution exhibited ahigher V value (1,603 m s−1) than pure water and the imogolite–based colloidal solution, which may originate from thehydration of maleic acid molecules by water molecules. In
addition, it is reasonable that the V value of the imogolite–
maleic-acid gel (liquid state) fell between those of the imogolite–based colloidal solution and the maleic acid solution.
Finally, we measured the temporal change in the ultrasonicvelocity of the imogolite–maleic-acid gel (0.04 mol L−1 inthis experiment), which shows gelation within 1 h [Shikinaka et al., 2016]. Figure 4 shows the output waveforms of
the receiver when the ultrasonic pulses transmitted throughthe thixotropic gel at liquid state (t=3 min) and gelated state(t=3.4 h). The two output waveforms were almost identical,
which indicates that the ultrasonic velocity in the thixotropic
gel is not largely affected by the liquid/solid transition. This
result is in good agreement with that previously reported(Norisuye et al., 2006).

K. Shikinakaa et al.

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CONCLUSION
The ultrasonic velocities in imogolite-based colloidal solution and thixotropic hydrogel were measured in this study.
The velocity of pure water was found to change after imogolite was mixed. In contrast, the ultrasonic velocity did notshow any dependence on the gelation of the imogolite-based
colloidal solution caused by its crosslinking via maleic acid.
The obtained results pave the way for the detection of diluted
clay minerals in solvents using ultrasonic waves. Furthermore, the experimental results show that the ultrasonic velocity differs depending on the molecular species (i.e., imogolite
or maleic acid) and the mixing of several molecules (i.e., hydrogel consisting of imogolite and maleic acid). This featureprovides a new possibility for the detection of species and theamount of clay minerals by ultrasonic velocity measurement
even in coexistence with other organic molecules, whichcannot be performed by other noncontact measurement techniques such as dynamic light scattering in the diluted state ofclay minerals.
ACKNOWLEDGEMENTS
This work was supported by the JSPS KAKENHI underGrant Numbers 18K05252, 16J09080, 25709032, 26630180,
and 17H04930.
REFERENCES
Bergaya, F., Theng, B.K.G., and Lagaly, G. (2006) Handbook of Clay Science, 1st Ed. Elsevier Science, Netherlands.
Donkai, N., Inagaki, H., K ajiwara, K., Urakawa, H., and Schmidt, M.
(1985) Dilute solution properties of imogolite. Die Makromolekulare
Chemie, 186, 2623–2638.
Farmer, V.C., Fraser, A.R., and Tait, J.M. (1977) Synthesis of imogolite:
a tubular aluminium silicate polymer. Journal of the Chemical Society.
Chemical Communications, 13, 462–463.
Farmer, V.C., A dams, M.J., Fraser, A.R., and Palmieri, F. (1983) Synthetic
imogolite: properties, synthesis, and possible applications. Clay Minerals, 18, 459–472.
Gong, F., Di, B., Wei, J., Ding, P., Pan, C., and Zu, S. (2018) Ultrasonic velocity and mechanical anisotropy of synthetic shale with different types ofclay minerals. Geophysics, 83, MR57–MR66.

H ayashi, N., Mizuno, Y., Koyama, D., and Nakamura, K. (2011) Measurement of acoustic velocity in poly(methyl methacrylate)-based polymer
optical ﬁber for Brillouin frequency shift estimation. Applied Physics
Express, 4, 102501.
H ayashi, N., Mizuno, Y., Koyama, D., and Nakamura, K. (2012) Dependence of Brillouin frequency shift on temperature and strain inpoly(methyl methacrylate)-based polymer optical ﬁbers estimated byacoustic velocity measurement. Applied Physics Express, 5, 032502.
Hübschen, G. (2016) Ultrasonic techniques for materials characterization.
p.177–224 in: Materials Characterization Using Nondestructive Evaluation (NDE) Methods (G. Hübschen, I. Altpeter, R. Tschuncky, and H.-G.
Herrmann, editors). Woodhead Publishing, Amsterdam.
K rautkrämer, J. and K rautkrämer, H. (1990) Ultrasonic Testing of Materials. Springer-Verlag Berlin Heidelberg, 677 pp.
Kuila, U. and Prasad, M. (2010) Pore size distribution and ultrasonic velocities of compacted Na-montmorillonite clays. SEG Technical Program
Expanded Abstracts 2010, 2590–2594.
Lubbers, J. and Graaff, R. (1998) A simple and accurate formula forthe sound velocity in water. Ultrasound in Medicine & Biology, 24,1065–1068.
Minakawa, K., H ayashi, N., Mizuno, Y., and Nakamura, K. (2013) Potential
applicability of Brillouin scattering in partially chlorinated polymer
optical ﬁbers to high-precision temperature sensing. Applied Physics
Express, 6, 052501.
Norisuye, T., Strybulevych, A., Scanlon, M., and Page, J. (2006) Ultrasonicinvestigation of the gelation process of poly(acrylamide) gels. Macromolecular Symposia, 242, 208–215.
Povey, M.J.W. (1997) Ultrasonic Techniques for Fluids Characterization.
Academic Press, San Diego, 214 pp.
Shikinaka, K., K aneda, K., Mori, S., M aki, T., M asunaga, H., Osada, Y.,
and Shigehara, K. (2014a) Direct evidence for structural transition
promoting shear thinning in cylindrical colloid assemblies. Small, 10,
1813–1820.
Shikinaka, K. and K imura, H. (2014b) Reversible viscosity change of nanotubular colloidal aqueous suspensions responding to an electric ﬁeld.
Colloids Suf. A, 459, 1–3.
Shikinaka, K., Koizumi, Y., and Shigehara, K. (2015) Mechanical/opticalbehaviors of imogolite hydrogels depending on their compositions andoriented structures. Journal of Applied Polymer Science, 132, 41691.
Shikinaka, K., Mori, S., Shigehara, K., M asunaga, H., and Sakai, T. (2016)
Kinetics-dominated structure and stimuli responsiveness in the assembly of colloidal nanotubes. RSC Advances, 6, 52950–52956.
Vanorio, T., Prasad, M., Patella, D., and Nur, A. (2002) Ultrasonic velocity
measurements in volcanic rocks: correlation with microtexture. Geophysical Journal International, 149, 22–36.
(Manuscript handled by Jun Kawamata)